scispace - formally typeset
Search or ask a question

Showing papers in "Philosophical Magazine in 1994"


Journal ArticleDOI
TL;DR: In this article, a twinned 9R structure was found in precipitates slightly larger than 4nm in diameter, following transformation from b.c.c to c.c, indicating that further twinning must occur during growth in the 9R phase.
Abstract: Transmission electron microscopy and high-resolution electron microscopy investigations of the structure of Cu precipitates in the size range 4–30 nm were carried out as part of a wider investigation into Cu precipitation from thermally aged Fe-Cu and Fe-Cu-Ni ferritic model alloys. A twinned 9R structure was found to be present in precipitates slightly larger than 4nm in diameter, following transformation from b.c.c. Two twin-related 9R variants were observed in all the smallest 9R particles studied. The 9R precipitates were observed to grow subsequently as spherical, multiply twinned particles up to approximately 17 nm, indicating that further twinning must occur during growth in the 9R phase. At sizes larger than 17 nm, a second transformation to the more stable 3R structure takes place. Observations on these 3R particles indicate that, following transformation from 9R, the precipitates are untwinned and have a distorted f.c.c. structure. The particle-matrix orientation is close to, but not ex...

366 citations


Journal ArticleDOI
TL;DR: In this paper, a model for a crack lying along one of the interfaces of a thin ductile layer joining two elastic solids is proposed for the joint whose major parameters are the layer thickness, the elastic-plastic properties of material in the layer, and the work of separation and peak separation stress of the local interface fracture process.
Abstract: The contribution of plastic deformation to the effective work of fracture is computed for a crack lying along one of the interfaces of a thin ductile layer joining two elastic solids. A model is proposed for the joint whose major parameters are the layer thickness, the elastic-plastic properties of material in the layer, and the work of separation and peak separation stress of the local interface fracture process. A symmetric mode I loading of the joint is considered under conditions where the thickness of the layer and the extent of the plastic zone are small compared with the crack length. The crack growth resistance behaviour is computed, with special emphasis on the steady-state work of fracture. The role of the layer thickness in the development of the plasticity contribution to toughness is detailed. Plastic dissipation is fully realized for layers above a certain thickness, characteristic of a plastic zone dimension, and is negligible when the layer is thin relative to this dimension. Othe...

188 citations


Journal ArticleDOI
TL;DR: In this paper, a model of the interaction of an acoustic wave with dislocation dipoles and dipole-array approximations to veins and persistent slip bands (substructures) formed during metal fatigue is presented.
Abstract: A model is presented of the interaction of an acoustic wave with dislocation dipoles and dipole-array approximations to veins and persistent slip bands (substructures) formed during metal fatigue. The model predicts the generation of a substantial acoustic second harmonic that depends on the distance between the glide planes of the dipole pair, on the dipole density, and on the particular arrangement and volume fraction of dipoles in a given substructure of the fatigued solid. Experimental evidence which strongly supports the essential features of the model is presented for fatigued aluminium alloy 2024-T4.

167 citations


Journal ArticleDOI
TL;DR: In this article, the lattice and sub-boundary diffusion coefficients of aluminium self-diffusion were measured in single crystals of α-Al2O3 in the temperature range 1540-1697°C using 26Al as a radioactive tracer.
Abstract: Aluminium self-diffusion has been measured in single crystals of α-Al2O3 in the temperature range 1540–1697°C using 26Al as a radioactive tracer. Two diffusion mechanisms are involved: a lattice diffusion mechanism and a subboundary mechanism. Lattice and subboundary diffusion coefficients obey equations and respectively. The results show that lattice diffusion coefficients of aluminium are much lower than those determined earlier. The comparison with oxygen diffusion determined by other workers for the same specimens shows that oxygen and aluminium diffusion coefficients are of the same order of magnitude in the lattice but that aluminium diffusion coefficients are greater than oxygen diffusion coefficients in subboundaries.

117 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used high-resolution transmission electron microscopy (HRTEM) images of a bend of a planar defect and various kinds of atomic steps, whose ledges are parallel to the [1&1bar;0] direction.
Abstract: Detailed structural data of the {′113} interstitial defect or the rod-like defect in Si and Ge have been given based on firm experimental evidence by high-resolution transmission electron microscopy (HRTEM). We have found line interstitial defect structures, in which a few chains of additional (interstitial) atoms in the 〈110〉 direction are inserted in a perfect crystal without dangling bonds in the {′110} cross-section. We have also found various kinds of atomic steps, whose ledges are parallel to the [1&1bar;0] direction, in the extremely extended {′113} planar defect. The HRTEM images of a bend of a {′113} planar defect have been also presented. The interstitial defect structures mentioned above have been determined by HRTEM with image simulation. Based on the experimental evidence, we have shown that the observed structures such as the extremely extended part on {′113}, the bend, the steps and the disturbed structure in which the hexagonal structure with stacking faults exists have been consi...

103 citations


Journal ArticleDOI
TL;DR: In this paper, the core structure and core energy of a straight dislocation with arbitrary Burgers vector in an arbitrary glide plane in a crystal of arbitrary anisotropy were determined by describing the internal displacements by appropriate trial functions with a set of free parameters.
Abstract: In the original Peierls-Nabarro model the core structure of a dislocation is determined as the solution of an integrodifferential equation. This equation describes the balance between the forces resulting from the deformation of two elastic half-spaces and from a one-dimensional periodic lattice potential acting across the glide plane. A method is described here which allows the core structure and core energy to be obtained for a straight dislocation with arbitrary Burgers vector in an arbitrary glide plane in a crystal of arbitrary anisotropy, for which the displacement potential is represented by a two-dimensional Fourier series. This is accomplished by describing the internal displacements by appropriate trial functions with a set of free parameters whose value is then determined by minimizing the total energy. The method is applied to obtain the core configuration of a screw dislocation dissociated in a {111} plane of a f.c.c. lattice.

97 citations


Journal ArticleDOI
TL;DR: In this article, the authors measured the strain rate sensitivity of Ni3(Al, Hf)B over a range of temperatures T in the yield stress anomaly up to 500 K, and as a function of strain E and of crystal orientation.
Abstract: The strain rate sensitivity (SRS) β = δτ/δ (In σ) of the flow stress τ, has been measured for crystals of Ni3(Al, Hf)B over a range of temperatures T in the yield stress anomaly up to 500 K, and as a function of strain E and of crystal orientation. β obeys a Cottrell-Stokes low when plotted against τ - τy, where τy, is the yield stress at 0·01% strain. The slopes are about 1%, decrease with increasing T and are approximately independent of orientation. The results show that the yield stress and work-hardening rates (WHRs) are controlled by different mechanisms. The flow stress τ=τy + τh, where τh is due to work hardening. τy is a reversible function of T, but independent of pre-strain ∊ and σ τh is a function of ∊, T and σ; it controls the SRS of τ. At yield, both edge and screw dislocations propagate on (111), the latter via lateral glide of edge character superkinks which bypass screws locked by cross-slip on to (010), allowing source operation and slip lines to be formed rapidly. Work hardenin...

75 citations


Journal ArticleDOI
TL;DR: In this paper, the splitting of disclinations at grain boundary junctions in polycrystalline solids is examined theoretically and related to local amorphization, showing that such a process competes effectively with microcracks generation at triple junctions of grain boundaries, causing a plastification of deformed polycrystaline materials.
Abstract: Splitting of disclinations (decay of disclinations into small-power disclinations) at grain boundary junctions in polycrystalline solids is examined theoretically, and related to local amorphization. Energetic characteristics of the splitting process are revealed. It is shown that such a process competes effectively with microcrack generation at triple junctions of grain boundaries, causing a plastification of deformed polycrystalline materials.

75 citations


Journal ArticleDOI
Tetsuya Ohashi1
TL;DR: In this paper, the plastic slip process in crystals is described by a continuum mechanics technique using models for the behaviour of dislocations, and new models are established for the mean glide path distance of dislaps during single slip and multiple slip and comparisons are made for the strain-hardening characteristics accompanying those models.
Abstract: The plastic slip process in crystals is described by a continuum mechanics technique using models for the behaviour of dislocations. Some new models are established for the mean glide path distance of dislocations during single slip and multiple slips and comparisons are made for the strain-hardening characteristics accompanying those models. The spontaneous transition in the deformation curve of single crystals from stage I to stage II is naturally introduced through models of the mean glide path distance.

73 citations


Journal ArticleDOI
TL;DR: In this article, the mean depth of permanent damage in both cases lies in the range 100-400 nm, and the damage consists of regions of well defined dislocation loops on several slip systems, other regions with a very high density of irregular dislocation arrays, and occasional microcracks up to 500nm deep which do not always intersect the surfaces.
Abstract: Silicon substrate slices in (111) or (001) surface orientation have been machined in two ways: precision ground by a diamond abrasive wheel to a surface roughness R a ≈ 11 nm, or turned on a highly stiff single-point diamond turning machine to R a,≈0·5 nm Transmission electron microscopy of cross-sections of the machined surfaces has established the following The mean depth of permanent damage in both cases lies in the range 100–400 nm In the ground specimens the damage depth is very variable, and the damage consists of regions of well defined dislocation loops on several slip systems, other regions with a very high density of irregular dislocation arrays, and occasional microcracks up to 500nm deep which do not always intersect the surfaces Patches of amorphous silicon are also observed Beneath the turned surfaces the damage appears more homogeneous, consisting of dislocation loops predominantly on a single slip system in any given region of the specimen

69 citations


Journal ArticleDOI
TL;DR: In this paper, the origin of tetrapod shape arises from the formation of octahedral multiple twins due to the small c/a ratio of wurtzite ZnO, the presence of in version-type twin boundaries and a large growth anisotropy with respect to the crystal polarity.
Abstract: The characteristic morphology of tetrapod-shaped ZnO particles grown by oxidizing zinc vapour in air has been thoroughly interpreted by a growth process from octahedral multiple-twin embryos. The origin of the tetrapod-shape arises from firstly the formation of octahedral multiple twins due to the small c/a ratio of wurtzite ZnO, secondly the presence of in version-type twin boundaries and thirdly a large growth anisotropy with respect to the crystal polarity.

Journal ArticleDOI
TL;DR: In this article, the critical resolved shear stress (CRSS) of a DO19 compound with an off-stoichiometric composition of Ti-36·5 at.% Al has been studied as a function of orientation at temperatures from −196°C to room temperature in compression.
Abstract: Deformation behaviour of a DO19, compound with an off-stoichiometric composition of Ti-36·5 at.% Al has been studied as a function of orientation at temperatures from −196°C to room temperature in compression. The critical resolved shear stress (CRSS) for {1100} 〈11 20〉 prism slip and (0001)〈1120〉 basal slip increases with decreasing temperature. a dislocations introduced by prism slip have a tendency to align along their screw orientation at −196°C, but the tendency becomes less marked with increasing temperature so that at room temperature these dislocations do not necessarily align along any particular crystallographic orientations. The CRSS for {1121} 〈1126〉 pyramidal slip shows a minimum in the CRSS-temperature curve at a temperature T min between −196 and −100°C. Most 2c + a dislocations are in screw orientation at temperatures below T min where the CRSS increases with decreasing temperature, whereas at temperatures above T min, where the CRSS anomalously increases with increasing temperatu...

Journal ArticleDOI
TL;DR: In this paper, the elastic stresses in the region adjacent to a twin boundary (TB) subjected to uniaxial tension are carried out for different orientations of the tensile axis.
Abstract: Calculations of the elastic stresses in the region adjacent to a twin boundary (TB) subjected to uniaxial tension are carried out for different orientations of the tensile axis. It is found that the principal stresses reach a maximum when the tensile axis is a 〈111〉 direction and that their value strongly depends on orientation. In general, the presence of the twin tends to favour the early development of secondary and multiple slip on at least one side of the TB. For cyclic deformation at intermediate amplitudes, this result is supported by experimental findings on the role of annealing TBs in favouring persistent slip band nucleation and promoting higher local hardening rates adjacent to them. The fact that the TB “stress concentrationz” is found to vary significantly as a function of orientation suggests that, in twinned polycrystalline materials, texture is an important factor in determining the role of TBs under loading, and thus their consideration as actual grain boundaries.

Journal ArticleDOI
TL;DR: In this article, the contribution of climb to the plastic deformation of TiAl when deformed at elevated temperatures where significant ductility is realized has been assessed, and it is shown that the motion of dislocations with Burgers vectors, b, given by b = 1/2〈110] is achieved by either or both glide and climb, the degree to which these various mechanisms contribute depending on temperature and strain rate.
Abstract: This study has assessed the contribution of climb to the plastic deformation of TiAl when deformed at elevated temperatures where significant ductility is realized. It is shown that the motion of dislocations with Burgers vectors, b, given by b = 1/2〈110] is achieved by either or both glide and climb, the degree to which these various mechanisms contribute depending on temperature and strain rate. Thus, at 900°C and strain rates less than approximately 10−2s−1, climb is found to contribute rather significantly to plastic deformation. A model is proposed which involves both glide and climb; initially, the deformation substructure is developed by glide until the appropriate link length for significant contributions from climb has been established. Calculations of the strain rate afforded by climb indicate that this form of deformation can contribute significantly to the deformation of TiAl at high temperatures.

Journal ArticleDOI
TL;DR: In this paper, the orientation relationship between V and Al2O3 was determined and the geometrical misfit dislocations and interfacial stacking faults were analysed using selected-area diffraction patterns and HREM.
Abstract: V films were deposited on the (102) R-plane of sapphire by molecular beam epitaxy. Cross-sectional and plan-view specimens were prepared and the V–Al2O3 interface was investigated from different directions by conventional and high-resolution electron microscopy (HREM). It was found that the film grows in an epitaxial manner with respect to the substrate with a tilt angle of about 2°. Using selected-area diffraction patterns and HREM, the orientation relationship between V and Al2O3 was determined and the geometrical misfit dislocations and interfacial stacking faults were analysed. The misfit dislocations are a geometrical consequence of the large mismatch between interatomic spacings at the two sides of the V–Al2O3 interface and exist from the start of deposition. Because of the large lattice mismatch, the V–Al2O3 interface is expected to be incoherent; however, because of bonding between the atoms on the two sides of the interface, the atoms in the vicinity of the interface are displaced from t...

Journal ArticleDOI
TL;DR: In this paper, an exact closed-form solution for the energy of arrays of dislocations lying at the interface between a strained epitaxial layer and substrate that are elastically anisotropic and have the same elastic constants is obtained.
Abstract: Exact closed-form solutions are obtained for the energy of arrays of dislocations lying at the interface between a strained epitaxial layer and substrate that are elastically anisotropic and have the same elastic constants. Closed-form solutions are obtained both when all dislocations in an array have the same Burgers vector and when the sign of one or more of the components of the Burgers vector alternates within an array. The method of solution makes the consideration of multiple arrays particularly simple (i.e. sets of more than one array in the interface, each inclined to the others at some non-zero angle, e.g. a pair of orthogonal arrays). Also, an approximate treatment is given of the case in which the elastic constants of the substrate differ from those of the layer by a small perturbation. The method is based on performing a perturbation expansion about the solution for the homogeneous body. A correction term, accurate to first order in the perturbation, is thus obtained to account for th...

Journal ArticleDOI
TL;DR: In this article, a detailed investigation of the superplastic deformation of two grades of the Pb-62 Sn eutectic (where the compositions here and subsequently are in weight per cent) was conducted under identical experimental conditions of grain size (about 8 μm), temperature (390-420 K) and stress (0·1-10 MPa).
Abstract: A detailed investigation of the superplastic deformation of two grades of the Pb-62 Sn eutectic (where the compositions here and subsequently are in weight per cent) was conducted under identical experimental conditions of grain size (about 8 μm), temperature (390–420 K) and stress (0·1–10 MPa). The first grade (grade 1) is Pb-62 Sn containing 890 wt p.p.m. Cd while the second grade (grade 2) is very-high-purity Pb-62 Sn. The experimental results on grade 1 (Pb-62Sn doped with Cd) show the presence of a sigmoidal relationship between strain rate and stress. This sigmoidal behaviour exhibits firstly an intermediate-stress region (region II) which is characterized by a stress exponent n of about 1·7 and an activation energy Q similar to the activation energy Q gb anticipated for grain-boundary diffusion and secondly a low-stress region (region I), where n > 2·6 and Q > Q gb. By contrast, the experimental results on grade 2 (high-purity Pb-62 Sn) show that the characteristics of superplastic flow at...

Journal ArticleDOI
TL;DR: In this paper, a new method for the determination of residual stresses in plastically deformed polycrystalline materials is presented based on standard X-ray and neutron diffraction measurements of the interplanar spacings in different directions.
Abstract: A new method for the determination of residual stresses in plastically deformed polycrystalline materials is presented. This method is based on standard X-ray and neutron diffraction measurements of the 〈d hkl〉 interplanar spacings in different directions. Until now the estimation of the residual stresses was limited to those of the first-order but the method presented takes into account both the first-and second-order stresses.

Journal ArticleDOI
TL;DR: In this article, an expansion of the palladium lattice, increasing on decrease in the mean cluster size, has been observed, attributed to pseudomorphism, and the Debye-Waller factor was also determined.
Abstract: Palladium clusters have been epitaxially grown under ultra-high vacuum (UHV) on MgO(100) surfaces cleaved in situ. The lattice parameter was measured in situ by surface electron energy-loss fine-structure spectroscopy (SEELFS) above the palladium N23 edge. The size distribution of the clusters was obtained from ex-situ transmission electron microscopy. An expansion of the palladium lattice, increasing on decrease in the mean cluster size, has been observed. From the UHV in-situ measurements the lattice dilatation is attributed to pseudomorphism. With the SEELFS technique the Debye-Waller factor was also determined. An increase in the mean square displacement of the atoms was observed when the cluster size decreased; the latter was explained by the increase in the proportion of surface atoms and by the effect of the lattice dilatation.

Journal ArticleDOI
TL;DR: In this article, a dependent Bloch wave theory for inelastic scattering is adapted to predict back-scattered electron (BSE) contrast from perfect crystals, which is correlated with 300 keV data from a number of zone axes from spinel, chromia, silicon, aluminium and gallium arsenide.
Abstract: A dependent Bloch wave theory for inelastic scattering is adapted to predict back-scattered electron (BSE) contrast from perfect crystals. This is correlated with 300 keV data from a number of zone axes from spinel, chromia, silicon, aluminium and gallium arsenide. This theory is shown to supersede an independent Bloch wave theory for BSE contrast which is incapable of accounting for asymmetry across polar axes. The orientation dependence of thermal dechannelling of fast electrons is shown to be important. The BSE scattering potential is shown to approximate to a delta function scaled by the atomic number squared on each atom.

Journal ArticleDOI
TL;DR: In this article, a particular formulation of the three-dimensional finite element method is established for the elastic analysis of the triangular pyramidal indentation, and numerical analyses are carried out under several conditions to investigate the influences of the elastic properties of the material and the apical angle α of the triangle pyramid indenter on the loadisplacement (P-h) relationship.
Abstract: A particular formulation of the three-dimensional finite-element method is established for the elastic analysis of the triangular pyramidal indentation. Numerical analyses are carried out under several conditions to investigate the influences of the elastic properties of the material and the apical angle α of the triangular pyramidal indenter on the loadisplacement (P-h) relationship. The analytical results are expressed by P [dbnd] CE∗ (tan α)h 2, where E∗ is the plane strain modulus of the material and C is a geometrical factor equal to 1·02. The value of C is a factor of 1·60 larger than the theoretical value for a conical indenter, 2/π. The analytical results are evaluated by nano-indentation measurements on five glasses; one is as fused and the others are mechanically surface polished. The experimental geometrical factor obtained on the former glass agrees well with the calculated value, while those on the latter glasses are slightly higher. The experimental P-h curves of the mechanically su...

Journal ArticleDOI
TL;DR: In this article, a model based on the dissociation of lattice dislocations into interfacial dislocation was proposed to explain the attractive dislocation-particle interaction during the creep of dispersion-strengthened alloys.
Abstract: A model based on the dissociation of lattice dislocations into interfacial dislocations, when they enter the matrix-particle interface for climb bypass, is proposed to explain the attractive dislocation-particle interaction during the creep of dispersion-strengthened alloys. This model predicts a threshold stress for creep which depends on the particle radius, the interparticle spacing and the reduction in dislocation self-energy because of dissociation. A minimum energy reduction of about 36% is required through the dissociation to offset the increase in line length energy because of the local climb configuration. There is reasonable agreement between theoretical predictions and experimental data on threshold stresses. The model is also supported by transmission electron microscopy evidence of departure-side pinning and contrast at the matrix-particle interface.

Journal ArticleDOI
TL;DR: In this article, the deformation mechanisms responsible for flow stress dependence on temperature have been compared for two alloys with atomic compositions Ti[sbnd]48Al and Ti[SBnd]47·5Al[nbnd]2·5Cr.
Abstract: Comparisons of the deformation mechanisms responsible for the flow stress dependence on temperature have been made for two alloys with atomic compositions Ti[sbnd]48Al and Ti[sbnd]47·5Al[sbnd]2·5Cr. Two different high values of flow stress have been observed, namely at −196 and 420°C. The high flow stress values measured at low temperatures have been attributed to a spontaneous blocking mechanism of the superdislocations present. However, the latter disappear at high temperatures and the anomalous strengthening effect that occurs between room temperature and 420°C has been described by the twinning activity and emission of single ½ 〈110] dislocations. An increase in twin density with increasing temperature produces increased stress concentrations at twin intersections from which a higher density of single dislocation can be emitted. An increase in the forest density produces multiplication of cutting events, leading to an increase in jog density along the dislocation line. The latter appears to b...

Journal ArticleDOI
TL;DR: In this article, the general mechanics and energetics of the Bain transformation are presented, and a rational means of comparing the various types is made possible by defining the longitudinal stretch as the independent variable or degree of transformation.
Abstract: The general mechanics and energetics of the Bain transformation are presented. The Bain transformation takes a crystal from its b.c.c. configuration into its f.c.c. structure, or vice versa, by means of homogeneous axial deformations. The crystal remains b.c.t. on the transformation path, and different types of Bain transformation may be distinguished by the response of the transverse lattice parameters to incremental changes in the longitudinal lattice parameter. A rational means of comparing the various types is made possible by defining the longitudinal stretch as the independent variable or degree of transformation. It is shown that, among possible Bain transformations, the one that occurs under a uniaxial-loading environment has the lowest binding energy at any given stage of transformation. In addition, the lowest possible barrier energy for any Bain transformation occurs when the crystal passes through a special unstresssd tetragonal state that resides at a local energy maximum on the unia...

Journal ArticleDOI
TL;DR: In this paper, a theoretical analysis based on the Fourier transformation approach was developed for the frictional pull-out of a single fibre with a rough interface from an elastic matrix, where a Fourier series was used to express the amplitude function of the interfacial roughness, and solutions for the fibre pullout stress were obtained for two different boundary conditions: one models a matrix containing an array of unidirectional fibres and the other a single fiber-matrix system.
Abstract: A theoretical analysis, based on the Fourier transformation approach, has been developed for the frictional pull-out of a single fibre with a rough interface from an elastic matrix. A Fourier series is used to express the amplitude function of the interfacial roughness, and solutions for the fibre pull-out stress are obtained for two different boundary conditions: one models a matrix containing an array of unidirectional fibres and the other a single fibre–matrix system. The interfacial roughness between the fibre and matrix is found to have a pronounced influence on the fibre sliding behaviour.

Journal ArticleDOI
TL;DR: In this article, the authors examined yield stress and ageing discontinuities in a Fe-43% Al alloy after carefully ageing in an attempt to control the number of excess vacancies and the form of their annihilation aggregates.
Abstract: Yield stress and ageing discontinuities have been examined in a Fe-43% Al alloy after carefully ageing in an attempt to control the number of excess vacancies and the form of their annihilation aggregates. Vacancies, probably present as submicroscopic aggregates, can act as strengthening obstacles to subsequent dislocation movement. When the vacancies are present in the form of somewhat coarser aggregates or dislocation loops they may act both as dislocation obstacles and vacancy sources during subsequent deformation at intermediate temperatures. The strengthening obtained depends sensitively on the precise vacancy aggregate present. In addition, such vacancy aggregates can lead to significant dynamic dislocation locking and the appearance of both static and dynamic strain ageing phenomena.

Journal ArticleDOI
TL;DR: In this article, the Peierls-Nabarroar model is used to calculate the critical applied stress beyond which the stability of the system breaks, and the magnitude of τp of the discrete model is larger than that of the continuum model for the same sinusoidal potential.
Abstract: The Peierls stress is calculated for a discrete Peierls—Nabarro model of a dislocation. Unlike the original continuum model where a continuous distribution of infinitesimal dislocations was considered, the discreteness of the slip plane is maintained throughout the calculation, and the Peierls stress τp is determined as the critical applied stress beyond which the stability of the system breaks. Results for three types of interatomic shear potential are well approximated by the relation τp G∞ exp(−Ah/b), as predicted by the continuum model, G being the shear modulus, b the spacing between slip planes, b the length of the Burgers vector and A a constant depending on the potentials. The magnitude of τp of the discrete model is larger than that of the continuum model for the same sinusoidal potential. Long-range potentials give low τp although they are still larger than experimental values.

Journal ArticleDOI
TL;DR: In this paper, the local lattice parameters and oxygen densities in the vicinity of large-angle grain boundaries in bulk YBa2Cu3O7−δ were measured simultaneously using convergent-beay electron diffraction and electron-energy-loss spectroscopy with a less than 20 A field emission probe respectively.
Abstract: Local lattice parameters and oxygen (hole) densities in the vicinity of large-angle grain boundaries in bulk YBa2Cu3O7−δ were measured simultaneously using convergent-beay electron diffraction and electron-energy-loss spectroscopy with a less than 20 A field emission probe respectively. The oxygen (hole) contents were quantified by Gaussian fitting to the observed oxygen K-edge absorption spectra and comparing them with results for a set of standard samples with different oxygen contents. The local lattice parameters were determined by matching the simulated high-order Laue zone patterns with experimental patterns by a least-squares algorithm. We found that large-angle grain boundaries in YBa2Cu3O7−δ often exhibit volume expansion. The increase of the c-lattice parameter near the boundary core can be attributed to lattice distortion and/or to an oxygen deficiency. The lattice distortion extends less than 20 A, while the deficiency can extend more than 100 A. Comparison of the c/a ratio of the lat...

Journal ArticleDOI
TL;DR: In this article, an approximate self-consistent scheme is presented for calculating the stress evolution in the individual grains of a polycrystal subjected to constant load conditions (creep), from initial loading to steady state.
Abstract: An approximate self-consistent scheme is presented for calculating the stress evolution in the individual grains of a polycrystal subjected to constant load conditions (creep), from initial loading to steady state. This formulation couples elastic, creep and growth strains, and provides a very flexible framework for treating the time-dependent response of anisotropic nonlinear visco-elastic polycrystals. It explicitly includes the anisotropy in the mechanical and thermal properties of the individual grains and of the polycrystal, and it accounts for intergranular interactions. The formulation is applied to the problem of irradiation creep (linear) and of thermal creep (nonlinear) of a reactor pressure tube, subjected to internal pressure. The effect on the overall response of the aggregate, of an irradiation-induced stress-independent deformation rate (growth) at the grain level is also considered. In a separate application, the coupling between the creep and the growth mechanisms in a non-textur...

Journal ArticleDOI
TL;DR: In this article, a detailed analysis of the dislocation-impurity interaction parameters of polycrystalline titanium alloys was carried out, taking into account the variations in the shear modulus and internal stress with temperature and the fact that impurity distribution along dislocations can have various statistics.
Abstract: Detailed thermal activation analysis has been carried out on experimental data for the plasticity of polycrystalline titanium containing 0·06–1·6 at.% O in the temperature range 4·2–600 K. The analysis of the dislocation-impurity interaction parameters took into consideration the variations in the shear modulus and internal stress with temperature and the fact that the impurity distribution along dislocations can have various statistics. Values of the energy and maximum force of the dislocation-impurity interaction, the “depth” of the obstacle (the interaction region), the pre-exponential factor in the Arrhenius equation, as well as the average dislocation segment length and the internal stress, have been determined. It has been shown that in the range below 550 K the kinetics of plastic strain in Ti-O alloys are controlled by thermally activated motion of dislocations through barriers formed by oxygen atoms. The specific features of the change in the plasticity parameters of α-Ti usually observe...